The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure is related in general to wellbore-telemetry technology. In particular, it relates to means by which the setting time of a cement slurry in a wellbore may be determined remotely.
After a well is drilled, the conventional practice in the oil and gas industry consists of lining the well with a metal casing. An annular area is thus formed between the casing and the subterranean formation. A cementing operation is then conducted with the goal of filling the annular area with cement slurry. After the cement sets, the combination of casing and set cement strengthens the wellbore and provides hydraulic isolation between producing zones through which the well penetrates.
It is common to employ more than one string of casing in the wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or a liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or hung off of the existing casing. Afterwards, the second casing string is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
The well cementing process typically involves the use of wiper plugs. Wiper plugs are elongated elastomeric bodies that are used to separate fluids as they travel through the casing interior. This practice prevents performance difficulties that may occur if the various fluids commingle. Usually the cementing operation requires two wiper plugs. When the cement slurry is ready to be dispensed, a first plug is released into the casing. The cement slurry is pumped behind the plug, thereby moving the plug downhole. At the bottom of the casing string, the first plug seats against a float valve, thereby closing off flow through the float valve. Hydraulic pressure builds above the first plug until it is sufficient to cause a membrane in the first plug to rupture. Thereafter, cement slurry flows through the first plug and the float valve, and up into the annular space between the wellbore and the casing string.
After a sufficient amount of cement slurry has been placed into the wellbore, a second wiper plug is deployed. A displacement fluid is pumped behind the second plug to move the second plug down the casing string, whereupon it lands upon the first plug. Unlike the first plug, the second plug does not have a membrane; therefore, the second plug seals the interior of the casing from the annular space between the wellbore and the casing string.
A thorough discussion of the primary cementing process may be found in the following publication: Piot B. and Cuvillier G.: “Primary Cementing,” in Nelson E. B. and Guillot D. (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006): 459-501.
After completing the cement-placement process, it is usually necessary to delay further wellbore operations for a time sufficient to allow the cement slurry to set and gain sufficient strength. Strength, is required to support the casing and provide zonal isolation. This idle time period is usually called the “waiting-on-cement” or WOC time. Traditionally, operators estimate the WOC time from pre-job laboratory testing. The cement slurry is placed in a pressurized curing chamber, and the time necessary for the slurry to set and develop sufficient strength is measured. Such measurements may be performed by crushing set-cement cubes after certain curing periods, or by continuously monitoring the sonic transit time through the cement slurry and calculating its strength.
The curing conditions for laboratory testing are usually estimated from field data provided by the well operator, temperature schedules based on the geothermal gradient, or both. The accuracy of these data are subject to several variables, including the temperature of the cement-slurry ingredients at the surface and the duration of time during which the well has been circulated and cooled before the cement job. The uncertainty may be further exaggerated when the casing string or liner is very long, resulting in a significant temperature gradient between the top and bottom of the string.
Underestimating the WOC time, and re-commencing wellbore operations prematurely may result in well damage, loss of zonal isolation or both. Overestimating the WOC time may result in unproductive rig time. In certain remote environments such as offshore platforms, rig time is usually very expensive, and halting well operations for an excessive period of time may be costly.
As a result, despite valuable contributions in the art, means by which the WOC time may be determined directly and unambiguously would be valuable. Such means would give operators more confidence when deciding to recommence wellbore operations.